Restriction of the use of certain hazardous substances (RoHS) in electrical and electronic equipment was seriously considered in a number of European Union (EU) legislations and directives (e.g. Marketing of Products Package, RoHs, EuP, etc.) so as to contribute to the protection of human health and environmentally sound recovery and disposal of waste electrical and electronic equipment (WEEE). The last proposal on RoHS (December 2008) sets a stricter ban of hazardous substances for a wider scale of applications. Particularly in medical devices and monitoring and control instruments, the use of lead in electronic ceramic parts (by more than 0.1 weight %) is not an exemption any more after 1Jan. 2014. However, up to date most of the high Performance piezoelectric devices (sensors, actuators, resonators and so on) have a lead-containing ceramic part. Typically the piezoelectric composition bases on the solid solution of lead zirconate titanate (PZT) and is in the vicinity of the morphortropic phase boundary (MPB) between lead titanate (PbTiO3) and lead zirconate (PbZrO3). The toxic element, lead (Pb), has a content of more than 60 weight % in these materials and thus leads to serious environmental problems both in producing and in post-treating of related WEEEs.
Numerous investigations have been done all over the world particularly in the last 10 years in order to find environmentally friendly substitutions for PZT. Unfortunately a real lead-free material which can completely replace PZT, both in terms of technical performance and in terms of production cost, has not been developed yet. Optimized compositions/structures/techniques have brought about some comparable piezoelectric properties corresponding to specific applications where large electro-strain is not a crucial issue. However, reported materials targeting actuation applications usually have a lower electrostrain response in comparison to that of soft PZT.
With the progress of materials science and engineering, piezoelectric properties of environmentally friendly piezoelectric ceramic materials have been greatly enhanced in the last two decades by optimizing composition, microstructure and processing related parameters. Three important perovskite families, namely barium titanate (BaTiO3-BT), potassium sodium niobate (K0.5Na0.5NbO3-KNN), and bismuth sodium titanate (Bi0.5Na0.5TiO3-BNT) have been intensively studied.
Barium titanate is one of the best known ferroelectrics, and achievements in improving its piezoelectric performance have been attained mostly by tuning the ferroelectric/ferroelastic domain configurations:    a) by fabricating specially oriented ceramics or single crystals, the anisotropic effect of the intrinsic piezoelectric properties can be used (maximum d33 of 203 pC/N);    b) by special poling treatment on single crystals, or using advanced sintering technology, the non-180° domains can be refined down to sub-micron scale, and the extrinsic contribution of their boundaries to piezoeffect be strongly enhanced (maximum d33 of 500 pC/N);    c) using random field defects (e.g. acceptor substitution and oxygen vacancy couples), the crystal or ceramic can be completely poled and re-depoled by applying and removing electric field. Ultrahigh electro-strain is thus caused by completely reversible domain switching. However, limited by the relative low Curie temperature, TC(˜130° C.), use of this material family is nearly excluded from high performance actuators.
Potassium sodium niobate (K0.5Na0.5NbO3) is a composition close to the MPB between potassium niobate and sodium niobate and has a high TC of more than 400° C., but its application has been strongly limited by the poor sinterability for a long time. Recently it was found that by introducing Li on A-site and Ta and/or Sb on B-site of the KNN lattice, the polymorphic phase boundary between tetragonal and orthorhombic phases can be shifted to the vicinity of room temperature, and with proper sintering aids the sinterability is greatly improved and piezoelectric properties greatly increased (maximum S33/E33 of 300 and 750 pm/V for random or oriented ceramics, respectively—Y. Saito et al. “Lead-Free Piezoceramics,” Nature, 432 [4] 84-7 (2004)). Even with optimized compositions and processing conditions, the improved piezoelectric properties of KNN-based ceramics is much lower than typical soft PZT, and their stability against temperature cycling degrades due to the orthorhombic/tetragonal phase transition. At the same time, textured ceramic processing routine has been developed to take advantage of the lattice anisotropicity. The textured ceramics have much better piezoelectric performance (which is comparable to soft PZT) but the complicated processing procedures and the increased cost practically prevent them from use in actuators.
Investigations on bismuth sodium titanate (Bi0.5Na0.5TiO3) ceramics have been focused on finding MPB compostions with other perovskites like BaTiO3, Bi0.5K0.5TiO3, K0.5Na0.5NbO3, and so on. Enhanced piezoelectric properties have been achieved (maximum d33 of 328 pC/N) but the best electrostrain response is still far below that of soft PZT. In addition, BNT-based materials usually have a low depoling temperature, Td, above which the polarization and/or piezoelectricity disappear due to the ferroelectric/antiferroelectric phase transition. Typically Td ranges from 100 to 250° C. and strongly limits the use of ferroelectric BNT-based ceramics in modern actuators. Texturing processes have also been studied but the improvement is not appreciable even disregarding the increased cost (maximum S33/E33 of 370 pm/V).
Recently it was reported that Td can be shifted down to below room temperature in properly modified ENT materials (S. T. Zhang et al. “Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3—BaTiO3—K0.5Na0.5NbO3 (BNT-BT-KNN) -system”, Applied Physics Letters 91, 112906 (2007)). In these compositions ultrahigh strain can be achieved due to the electric field induced ferroelectric phase transition. The effective piezoelectric constant (Smax/Emax) can be comparable (in ceramics maximum S33/E33 of 690 pm/V) or even superior to (in single crystals maximum S33/E33 more than 2000 pm/V) some of the soft PZT ceramics. Furthermore, the depoling temperature is not a practical limit any more as herein the piezoelectricity is not a necessity for the large strain behavior.